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Learn JS Data

Data cleaning, manipulation, and wrangling in JavaScript

Working with Strings

String cleaning is something you end up doing quite a lot. Hopefully this task will help make the process less painful. There are a near infinite transformations you might want to do with strings, so we won't get to everything, but this will serve as a starting point for common manipulations that will come up again and again.

We will start with generic JavaScript string functions and add in a bit of lodash magic to make things easier.

String Basics

Similar to arrays, the characters in strings are accessible via indexing

var aChar = "Hello There!"[6];

=> T

Also, just like arrays, you have access to the powerful slice method, which is used to extract sub-sections based on indexes.

var aSlice = "Hello There!".slice(6,11);

=> There

The sliced string goes up to - but not including - the last index.

And, of course, string concatenation is done in JavaScript using the + operator. Use parenthesis if you want to do actual arithmetic inside your concatenation.

var orderNum = 8;
console.log("You are number " + (orderNum + 1) + " in line.");

=> You are number 9 in line.

Check the documentation for all the other basic tools.

Stripping Whitespace

Often, you are going to have some surrounding whitespace that you don't want corrupting the rest of your data. Reading CSV files gives a good example of this, as spaces are typically also used in conjunction with the commas to separate columns.

A data file like this:


city  ,state ,population,land area
  seattle  ,WA , 652405 ,83.9
new york,NY,8405837,  302.6

When read in can produce quite the messy dataset:

d3.csv("data/cities_spaced.csv", function(data) {

=> [{"city  ":"  seattle  ","state ":"WA ","population":" 652405 ","land area":"83.9"},
{"city  ":"new york","state ":"NY","population":"8405837","land area":"  302.6"}]
This code is using d3.js

Note the spaces in the property names as well as the values. In cases like this, it might be best to map the data back to a clean version. Lodash's trim can help. It removes that unsightly whitespace from the front and back of your strings.

Here is a version of the data loading function that removes whitespace. It uses

d3.csv("data/cities_spaced.csv").then(function(data) {
  var clean = {
    var cleanD = {};
    d3.keys(d).forEach(function(k) {
      cleanD[_.trim(k)] = _.trim(d[k]);
    return cleanD;

=> [{"city":"seattle","state":"WA","population":"652405","land area":"83.9"},
{"city":"new york","state":"NY","population":"8405837","land area":"302.6"}]
This code is using d3.js and lodash

The strings are now clear of those pesky spaces.

Find and Replace

Extracting data from strings can sometimes mean extracting pieces of strings. Finding out if a string contains a keyword or sub-string of interest is a first step in quantifying the content of a body of text.

indexOf can be used to perform this searching. You pass it a sub-string, and it'll tell you the location in string you are calling it where that sub-string starts. -1 is returned if the sub-string can't be found. You can use this to build a little string finder, by comparing the return value to -1.

console.log("A man, a plan, a canal".indexOf("man") !== -1);

=> true
console.log("A man, a plan, a canal".indexOf("panama") !== -1);

=> false

Replace is the butter to find's bread. We will see more replacing when we get to regular expressions, but replacing sections of a string can be done with the replace method.

console.log("A man, a plan, a canal".replace("canal", ""));

=> "A man, a plan, a"


When you need to create a more complicated string, such as an html snippet, it may become too tedious to just combine strings by concatenating them with your variables. Consider the following example:

<div class="person">
  <span class="name">Birdman</span>
  <span class="occupation">Imaginary Super Hero</span>

If we wanted to build it using string concatenation, it might look like this:

var person = { name : "Birdman", occupation: "Imaginary Super Hero" };
var html_snippet = "<div class=\"person\">" +
  "<span class=\"name\">" + + "</span>" +
  "<span class=\"occupation\">" + person.occupation + "</span>" +

=> '<div class="person"><span class="name">Birdman</span><span class="occupation">Imaginary Super Hero</span></div>'

That's a lot of string escaping! You can imagine this gets pretty hard to manage after a while.

In order to simplify this process, you can use lodash templates to define a "template" that you can reuse with different data. Using our example above, we might define it like so:

var templateString = "<div class='person'>" +
  "  <span class='name'><%= name %></span>" +
  "  <span class='occupation'><%= occupation %></span>" +
var templateFunction = _.template(templateString);

Now you can use this template function with lots of data to generate the same snippet of html:


=> '<div class="person"><span class="name">Birdman</span><span class="occupation">Imaginary Super Hero</span></div>'
This code is using lodash
var anotherPerson = { name : "James. James Bond", occupation: "Spy" };

=> '<div class="person"><span class="name">James. James Bond</span><span class="occupation">Spy</span></div>'

Next Task

Regular Expressions

See Also